The aqueous dispersion copolymerization of tetrafluoroethylene (TFE) with perfluorinated alpha olefins such as hexafluoropropene (HFP) produces a raw product which must be further treated or "finished" before being melt fabricated into a useful final product. A major purpose of the "finishing" is to remove all sources of volatiles which could cause bubbles or voids in the final product.
Several sources of volatiles can exist in the unfinished or raw polymers including unstable endgroups and unstable backbone linkages. Various types of endgroups are possible but use of the common persulfate initiator leads to carboxylic acid endgroups. The degradation of these carboxylic acid ends can be a major source of volatiles at fabrication temperatures as was discussed in U.S. Pat. No. 3,085,083 (R. C. Schreyer). The polymer endgroups after extrusion may also be vinyl (--CF.dbd.CF.sub.2), difluorohydride (--CF.sub.2 H), internal olefin [--CF.dbd.CF(CF.sub.2)CF.sub.3,--(CF.sub.2).sub.n CF.sub.3, or carboxylic acid fluoride (--COF). The relative amounts of these ends will be dependent on the extrusion conditions, the type of initiator, the presence of salts which can catalyze the decarboxylation, and moisture level.
The second source of volatiles is believed to be due to the presence in the polymer backbone of relatively unstable linkages which thermally degrade during fabrication. These links may be present even if the polymer has stable endgroups directly from polymerization. The elimination of most of these links is desirable to achieve a low volatiles level. The structure(s) of the unstable backbone linkages in perfluorocopolymers have not been identified but appear to be comonomer diads not involving TFE. Thus in TFE/HFP copolymers the unstable backbone linkage arises from two adjacent HFP units. The volatiles arising from initial thermal degradation of TFE/HFP copolymer flake (unfinished) are very rich in HFP monomer. The level of unstable backbone linkages in HFP/TFE copolymers appears to be an inverse function of molecular weight (and melt viscosity). This suggests that the HPF diad forms by combination as the termination mechanism in polymerization. Regardless of the structure of the polymer backbone instability, a concentration can exist which causes problems with bubble formation during melt fabrication.